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Passive anti-dew system (i.e., requiring no power like a lens/corrector hood) may be able to help but only up to a certain extent (it delays the formation of dew, perhaps for a few hours, but will not completely stop it). As soon as the temperature of the scope drops below that of the dew point, dew will start to form and you will have no choice but to end your observation early. Without an active anti-dew system (heating by using electricity), it is simply impossible to completely eliminate dew. In this article, I will describe how an inexpensive DIY dew heater could be constructed using nichrome wire as the heating element.

DIY dew heater using nichrome wire

This problem may not be very evident on a reflector since its mirror is located at the bottom of the telescope’s optical tube, thus, the tube itself acts as a large anti-dew ‘hood’, unlike in telescopes with an optical element located at the front: a lens in the case of a refractor and a corrector plate for Schmidt-Cassegrain. Without a dew heater, it will only take a few hours before you notice the formation of dew (wiping the lens with a cloth is highly discouraged for the fear of damaging the delicate coatings).

Don’t you dare wipe it :)

WARNING: I will not be responsible for any damage caused to your equipment. Follow instructions at your own risk!

In this particular DIY, I have decided to use nichrome wire (instead of resistors) as the heating element. After building a resistor-based heater, I believe nichrome wire proved to be a simpler and more practical heating element. Nichrome wire is available in most hardware stores. The one I have used has a diameter of 0.27 mm.

A nichrome wire heats up when electricity passes through it, a physical property that makes it a material of choice for heating purposes

We will also need a flexible plastic strip (you can just cut one from a plastic folder), a shrinkable tube (optional, but it will make the heater professional-looking), some masking/electrical tape, scissors, some Velcro (optional), and a soldering iron.

Flexible plastic strip and shrinkable tube

It is also assumed that you already have a field battery (heaters are power-hungry!), and as an optional accessory should you wish to have a means of controlling the heater’s temperature, you need a power converter with variable output voltage (e.g., 1.5 to 12 V). (Note: I will write a separate DIY post for constructing a field battery.)

A 12V 36A field battery (left) and a converter with variable output voltage 1.5 to 12 V, 2A (right)

When electricity runs through a nichrome wire, it heats up. The shorter the wire, the hotter it gets (careful, it may even burn!). For the purpose of setting a safe temperature level, we need to determine the length of the nichrome wire that will produce a high temperature but still comfortable enough for you to hold (as warm as a typical coffee in a paper cup). This will ensure that even at maximum power, the temperature of the system never gets too high (otherwise, you’re risking burning your telescope!).

WARNING: The next part needs to be done with extreme care. You may burn your hands if you are not careful.

Tape a length of nichrome wire about 4 feet on to a table then connect its two ends to the power supply. As soon as the connection is made, the nichrome wire starts to generate heat. At a length of 4 feet, a 0.27 mm nichrome wire powered by a 12V 2A converter (or even directly via batteries) would not really generate much heat (it would be safe enough to hold). Let it warm up for 2 minutes and then sever the connection. You should be able to feel that it has increased its temperature a bit. Now try to shorten the wire by sliding the connector, perhaps by a few centimeters, then repeat the procedure described above. With each trial, you will notice that it becomes warmer and warmer. Since active heaters only need to raise the temperature of the lens/corrector plate by a couple of degrees, a ‘lukewarm’ or warm-water-hot-coffee-temperature should be more than enough to eliminate dew. Note that we don’t want it too cold either, since without enough heat, it would not be powerful against nights with exceptionally high amounts of dew! Stop when a comfortably warm temperature is achieved. Measure the length of wire. It should determine the minimum length of nichrome wire that we should use. In my setup, I used a 2-feet long nichrome wire.

Determining the appropriate length. As the red wire slides towards the left, the nichrome wire becomes hotter and hotter. Stop when a comfortably warm temperature is achieved.

You may calculate the resistance and then use an ohmmeter to determine the proper wire length. For instance if you have an 8-inch objective, Sky & Telescope suggests that we deliver 3 watts of power for an 8-inch lens/corrector plate and 1½ watts for a finderscope objective or eyepiece. The required resistance that will generate the desired heat is determined by the following formula:

Resistance needed = The square of the voltage of the power source divided by the amount of power or watts needed.

In symbols, R = V²/P (where R = resistance, V = volts, P = power)

For the 8-inch objective, we need 3 watts (as recommended by S&T). If you have a 12 V power supply, the required resistance would be:

You can then use an ohmmeter to determine the length of wire that would yield the required resistance.

Wrap the nichrome wire around the flexible plastic strip. Make sure that it is evenly-spaced and that no connection is bridged or shorted out. Use a strip of insulated wire (see red wire below) to bring the connection to the other end, then secure everything with an electrical tape. Slide the whole assembly onto the shrinkable rubber tubing (blue) as shown below:

A coil of nichrome wire

Devise some means of securing/attaching the heater on to the telescope. You may just use tape, but for easy attachment and removal of the heater, I opted to use Velcro in my setup.

DIY dew heater with 1 coil of nichrome wire. Temperature may be regulated by adjusting the voltage through the converter.

I have prepared a few more units which I intend to use to build a more powerful heater in case I’d be needing more heat, since exceptionally clear nights are the ones with the most dew! Each coil must have a length of nichrome wire that is no shorter than the minimum prescribed length.

Construct as many coils as necessary

By sewing a piece of cloth holder, it is possible to create a professional-looking heater. Connect each coil in parallel with each other.

Connect coils in parallel

Shown below is my DIY dew heater. It has 4 coils of wire which should be powerful enough to zap away any dew!

DIY dew heater with 4 coils of nichrome wire. Temperature may be regulated by adjusting the voltage through the converter.

I have prepared 2 more coils for my finderscope and my guidescope. These coils are taped directly on to the lens hood.

DIY dew heater for my finderscope (A and B) and guidescope (C).

A total of 7 coils (heating elements) were used in my setup. Four coils on the main imaging scope, 1 coil on the guidescope, and 2 on the finderscope. All these costed me less than 2 US dollars (approximately 100 Php), and only took me a couple of hours to build.

DIY dew heater: main scope (1), guidescope (2), finderscope (3 and 4), thermometer (5), field battery (6). Always use the lowest power setting to avoid affecting the optical performance of the telescope, as well as to conserve battery power.

I have been using this setup since November 2011 and have not had problems with dew. Should you have any questions, feel free to leave a comment. Clear skies!

For tutorials on how to get started with astrophotography, click here.

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11 Comments:

Nice article and it’s good to know that you applied the technique I taught you before on the use of the multiple voltage adapter as a means to control the heat on the dew heater :) LOL Good it worked for you as well. I also taught Oliver on this technique as well last year but he haven’t got the time yet to build his own dew heater till now :) LOL

Yes, that will be better but for practicality is concerned despite the fact that Im using now a commercially made dew controller, I think the main heater that needs control is the main lens. Rest no need. In Cali, mostly is set to high under clear skies thus IMHO is not that necessary as well on most occasion.

Hi !
You’ve resolved my problem and it has nothing to do with astronomy (but I like astronomy)
I needed something to keep my water bottle from freezing when I’m going winter camping. Your system will work perfectly! Thanks !

A very helpful blog. Thanks. Have you any experience of using a DIY dew heater with a camera lens? Does the heater warm the glass itself or just the surrounding air? If the heater protrudes in front of the camera lens, it may appear in the photos (my current interest is star trail images). Any advice appreciated.

Hello there
Very useful on making your own dew heater and field battery, I intend to make a couple dew heaters this weekend but i have one question.
When all the heaters are made on your setup are all the dew heaters connected togeather power supply if so how or are the all powered individualy.
many thanks.

These heaters are powered individually. Such arrangement allows me to control each coil individually, and save battery power by generating just enough amount of heat. It would also mean that for every coil, I use one voltage controller. In any case, if only one controller is available, you may opt to just connect the coils in parallel, and control the power output of the whole system using one voltage controller. If no controller is available, a switch for each coil should be sufficient enough, but you will have to monitor the temperature of the coils as they do sometimes get hot at full power.

I have a quick question though.
I’ve been using a lens heater made out of resistors (but it’s not ideal, it breaks easily) along with a PWM. Do you recommend me to keep using the PWM with my soon-to-be-built nichrome wire heater to control the voltage?

Hi, great tutorial. I’m looking to make something like this for my DSLR, mainly for shooting weddings in winter. The problem I have is entering a warm venue after doing shots in the cold outside. Given that I won’t need to power the heater for a long period, what capacity/type of battery do you think I could get away with? I want to keep it as portable as possible.

I believe what you want to achieve is to keep the camera (at least the lens) warm while you shoot outside, so that when you move from a dry and cold environment to a warm and humid one, there will be no drastic difference in temperature, and thus, no condensation will occur. In theory, it should work, for as long as the temperature of the camera body/lens while wrapped with a heater is almost the same as the temperature indoors. In the anti-dew heating system described above, the only requirement is to raise the temperature of the lens a few degrees above dew point (and not to make the lens warm, in fact, it still feels cold to touch), it consumes less power. Thus, the heater that you have in mind will consume more power since you need to match the temperature indoors. My suggestion would be to use the largest battery you are willing to carry (the one with the highest capacity), and then experiment with various heating coils (different resistance values, thus, generating different amounts of heat). Once you have found a length of coil powerful enough to keep the camera warm, you can then use smaller batteries for shorter outside shoots and also make your setup a bit lighter. :) Another option would be to simply allow 20-30 minutes for the camera to adjust to the new environment, following this method. Good luck!